1. Field of the Invention
This invention relates to a control system for an automatic transmission having a lock-up clutch.
2. Description of Related Art
An automatic transmission system is generally provided with a torque converter and a multiple stage transmission gear mechanism connected with an output shaft of the torque converter. The automatic transmission system is further provided with a control device for controlling the multiple stage transmission gear mechanism to provide a proper shift gear stage among a plurality of shift gear stages thereof in accordance with a running condition of vehicle. The torque converter is provided with a lock-up clutch for connecting an input member with an output member without intervention of a hydraulic fluid. In a relatively high speed and stable running condition of the vehicle, the lock-up clutch is engaged to connect the input member with the output member of the torque converter directly so as to improve fuel consumption performance. The control device is provided, as shift mode patterns with an economy mode in which a shift operation is made at a relatively low speed of the vehicle and with a power mode in which the shift operation is made in a relatively high speed condition. The control device selects one of the shift mode patterns in light of the driver's taste. A slip control of the lock-up clutch has been also known wherein the lock-up clutch is controlled in a slip control zone to produce a certain slippage between the input and output members.
In an automatic transmission system disclosed by Japanese Publication No. 51-22698, published for opposition in 1976, when an acceleration requirement is detected, the shift mode is switched from the economy mode to the power mode to improve acceleration performance. Japanese Patent Public Disclosure No. 58-37368, laid open to the public in 1983, proposes that the lock-up clutch be disengaged to improve the acceleration performance in an accelerating condition of the vehicle.
It should, however, be noted that the conventional control system for the automatic transmission simply Provides a switching action on the shift mode and the lock-up. These controls are not responsive to the running condition of the vehicle.